One of the most successful seal means used in earth-boring bits of the type having rotatable cutters is the O-ring seal disclosed in commonly assigned U.S. Pat. No. 3,397,928, to Galle. The o-ring seal successfully confines lubricant to the bearing area while excluding detritus for long periods of time before failure.
A more recent seal development is the rigid or metal face seal. In the rigid face seal type, the seal interface is between one or two rigid, usually steel, seal rings. One or two elastomer o-rings serve to energize or urge the seal faces of the rigid ring or rings in contact with each other. The rigid face seal has proved to be as successful as the o-ring seal and provides improved durability in high rotational speed applications.
However, because the seal faces of rigid face seals are in constant contact and slide relative to each other, the dominant mode of failure of the seals is wear. Eventually because of the wear, the seal face geometry changes such that the ability of the seal to maintain sealing effectiveness is lost. This leads to eventual seal failure and ultimately results in bit failure. In an effort to minimize seal wear, rigid face seal rings of prior-art seals are constructed of metal alloys such as 440C stainless steel or cobalt base alloys such as Stellite. Use of these materials for the rigid face seals has resulted in significantly increased bit life, but additional improvement in the seal durability is desirable to extend the life in the most severe applications.
Super-hard materials such as natural and synthetic diamond materials have been used on cutting elements for drill bits for some time. It is also known to utilize polycrystalline diamond (PCD) on cutting elements on drill bits of both the fixed cutter and rolling cone type. Also, PCD is used for thrust bearings for downhole drilling motors. The individual PCD disks are spaced in a circular array around the face of a shaft. The PCD diamond material is usually formed in high temperature and high pressure conditions (“HTHP”) in which the super-hard material is thermodynamically stable. This technique is conventional and known by those skilled in the art. In the most common process, diamond powder is placed in a refractory vessel. A sintered tungsten carbide disk is placed on the diamond powder. The contents of the vessel are then subjected to high pressure and temperature.
Silicon bonded PCD disks are also available, such as described in U.S. Pat. No. 4,793,828. A silicon bonded PCD disk has a mass of diamond particles present in an amount of 80 to 90 percent by volume and a second phase present in an amount of 10–20 percent by volume. The mass of diamond particles contains substantial diamond-to-diamond bonding to form a coherent, skeletal mass. The second phase consists essentially of silicon, the silicon being in the form of silicon and/or silicon carbide.